Unmanned Aerial Vehicle (UAV) activity has become increasingly common in airspace around the world. The increase in activity is driven by many types of users, including noncommercial-type entities (i.e., hobbyists, etc.), commercial-type entities (i.e., aerial photography, surveying, etc.), and military entities. Generally UAVs operate at low altitudes where air traffic is busiest and most unpredictable. For example, take-off and landing of commercial aircraft, test flights, private pilot activity, hobbyists, balloons and blimps, aerial advertising, float planes, emergency responders, and other UAVs may be more likely to be present within typical UAV airspace. A UAV operating autonomously or under the control of an operator must actively avoid interference with other UAVs operating in a common airspace.
Currently, aircraft operating in regulated airspace, such as airspace regulated by the United States' Federal Aviation Administration (FAA) or the European Union's European Aviation Safety Agency (EASA), may be required to be equipped with a Traffic Collision Avoidance System (TCAS) depending the aircraft type and type of operation. TCAS systems are often heavy, expensive, and/or rely on active interrogation of the transponder of an aircraft in the vicinity of the interrogating aircraft. Lighter systems are generally passive and rely on periodic transmission of transponder information from nearby aircraft, thereby only passively preventing an interaction between aircraft in the vicinity of the transmitting aircraft. In some instances, an object may not be equipped with a transponder and therefore would be invisible to passive detection using these techniques. Additionally, in the busiest and most unpredictable airspace, i.e., low altitudes, manned air vehicles typically rely on the pilot and Air Traffic Controllers to prevent interactions and maintain adequate separation between aircraft.